This objective covers how routers share map information with each other, make critical decisions on which path to choose when multiple routes exist, and manage IP translations and failover to keep traffic moving.
1. Static vs. Dynamic Routing
Routers need a map (a routing table) to know where to send packets. There are two primary ways to build this map:
- Static Routing: A network administrator manually types the route into the router’s configuration.
- Pros/Cons: Uses zero CPU processing power or bandwidth, making it highly secure and predictable. However, if a cable breaks, a static route cannot adapt; it will blindly forward traffic over the dead link until an administrator manually changes it.
- Dynamic Routing: Routers use specialized protocols to communicate with their neighbors, automatically sharing network maps, discovering new paths, and adapting instantly when a link goes down.
Core Dynamic Routing Protocols
- OSPF (Open Shortest Path First): An open-standard Link-State protocol used inside a company’s internal network (an Interior Gateway Protocol). It divides a network into “Areas” (with Area 0 being the mandatory backbone) and uses the Shortest Path First algorithm to map the quickest path based on link bandwidth.
- EIGRP (Enhanced Interior Gateway Routing Protocol): An advanced Distance-Vector protocol developed by Cisco (though now partially open standard) used internally. It is incredibly fast at adapting to changes because it precalculates backup paths (known as feasible successors) before a failure occurs.
- BGP (Border Gateway Protocol): The ultimate Path-Vector protocol that literally runs the global Internet. It is an Exterior Gateway Protocol used to route traffic between different massive networks (Autonomous Systems) owned by ISPs, universities, and tech giants.
2. Route Selection (The Tie-Breakers)
What happens when a router learns three different paths to the exact same destination? It uses a strict corporate chain of command to pick the winning route.
The router checks these three criteria in this exact order:
1. Prefix Length (The Specificity Rule)
The router will always choose the most specific route first (the one with the longest subnet mask/CIDR notation). This overrides everything else.
- Example: If a router has a path to
10.0.0.0/24and another path to10.0.0.0/16, it will choose the/24path because it is a more precise match for the destination.
2. Administrative Distance (The Trustworthiness Rule)
If the prefix lengths match exactly, the router compares how the route was learned. Administrative Distance (AD) is a predefined rating of how trustworthy a routing source is. The lower the number, the more trusted the route.
You should recognize these default AD values for the exam:
- Directly Connected Network:
0(Most trusted) - Static Route:
1 - EIGRP Route:
90 - OSPF Route:
110 - External BGP Route:
20
3. Metric (The Cost Rule)
If the prefix lengths match AND the routes were learned using the exact same protocol (meaning the AD is a tie), the router looks at the Metric. The metric is the “cost” calculated by that specific protocol to reach the destination. The lower the metric, the better the path.
- OSPF calculates its metric using Cost (based entirely on bandwidth).
- EIGRP calculates its metric using a complex formula that combines Bandwidth and Delay.
3. Address Translation: NAT vs. PAT
Address translation allows private internal networks to communicate out onto the public internet using a limited pool of public IP addresses.
- NAT (Static / Dynamic Network Address Translation): Maps private IP addresses to public IP addresses. In its simplest form (Static 1:1 NAT), one private IP address maps directly to one public IP address. This is often used to expose an internal web server to the public internet under a specific public IP address.
- PAT (Port Address Translation / NAT Overload): This is what you use at home and in almost all offices. It allows thousands of internal private devices to share a single, solitary public IP address simultaneously. The router modifies the source port number of every outgoing packet so it can keep track of exactly which internal device requested which website.
4. Gateway Redundancy & Routing Tricks
FHRP (First Hop Redundancy Protocol) & Virtual IP (VIP)
If your office building has only one router serving as the default gateway and that router catches fire, the entire building loses internet access. FHRP allows you to group multiple physical routers together into a single team to prevent a single point of failure.
- How it works: You configure two physical routers to run an FHRP protocol (like HSRP or VRRP). Instead of pointing your workstations to a real router’s physical IP, you point them to a shared Virtual IP (VIP) managed by the protocol.
- One physical router acts as the “Active” gateway handling traffic. If it dies, the “Standby” router instantly assumes control of the Virtual IP within seconds. The end-user devices never notice a drop in service because their default gateway IP destination never changes.
Subinterfaces (Router on a Stick)
Normally, a router interface can connect to only one physical network or VLAN. A subinterface is a logical software split created on a single physical router port.
This allows you to take one single physical cable running from a switch trunk port to a router, split that router interface into virtual subinterfaces (e.g., Gig0/0.10 for VLAN 10 and Gig0/0.20 for VLAN 20), and route traffic cleanly between multiple internal VLANs over a single physical link.
Key Exam Study Matrix
| Concept | Key Indicator / Clue | Primary Advantage |
| BGP | Autonomous Systems / Internet scale | Connects ISPs and large external global networks. |
| Prefix Length | Longest subnet mask match | The #1 absolute tie-breaker in routing selection. |
| AD (Admin Distance) | Trustworthiness rating (0–255) | Ties breaker between different protocols (Lower = Better). |
| PAT | Source port tracking / Overloading | Saves public IPs; allows a whole office to share one public IP. |
| FHRP | Virtual IP (VIP) / Active-Standby | Eliminates the default gateway’s single points of failure. |
| Subinterfaces | Logical slice of an interface | Enables inter-VLAN routing over a single physical cable link. |